POLYMER DISSOLUTION .1. ON THE DISSOLUTION BEHAVIOR OF COPOLYMERS OF METHYL-METHACRYLATE AND METHACRYLIC-ACID

Copolymers of methyl methacrylate with methacrylic acid [P(MMA/MA)] ar e interesting resist materials for microlithography. At moderate bakin g temperatures, these copolymers undergo an intramolecular cyclization , yielding terpolymers containing anhydride moieties. This process has striking consequences for the dissolution behavior and results in hig her ketone solubilities. The dissolution rates in methyl ethyl ketone (MEK) and in mixtures with ethyl glycol (EG) correlate in an Arrhenius -like manner with the reciprocal baking temperatures. Two distinct tem perature ranges with different slopes between 130-180-degrees-C and 18 0-230-degrees-C are found. This is in good agreement with other findin gs, indicating a different mechanism of anhydride formation in these t emperature regions. The activation energies for the dissolution of P ( MMA/MA) in EG or EG/MEK mixtures are about 17 kcal/mol and those of th e thermally treated material in MEK or MEK/EG or mixtures of MEK with methyl isobutyl ketone (MIBK) are in the range of 20-30 kcal/mol. For the investigation of the M(n) dependence, gamma-irradiated probes of t he copolymer were used. We obtained the usual exponential M(n) depende nce with an exponent of 0.7-0.8 in MIBK and 0.3-0.4 in EG. Our finding s are in agreement with a ''relaxation-controlled'' dissolution behavi or, especially for the anhydride-containing terpolymer. No residual la yers or pronounced inhibition periods indicative for gel-layer formati on, however, could be found. We suggest a normal dissolution process w ith a very small gel layer. For the copolymer in an alcohol-containing solvent, a stress-driven dissolution behavior is more likely. (C) 199 4 John Wiley & Sons, Inc.